Stents are medical devices commonly used to maintain patency of body vessels, such as those of the vascular and gastrointestinal systems. Stents are often delivered via a minimally invasive procedure and thereafter expanded to contact and support the inner wall of the targeted vessel. In general, most stents include a tubular shaped support structure having a plurality of interstices configured to facilitate compression and expansion of the stent.
In some instances, it is advantageous for a stent to include a covering over its support structure. Esophageal stents, for example, are often encased in a silicone membrane to prevent tumor ingrowth and overgrowth, to seal fistulas, and to reduce food impaction as well as tissue perforation. Covered stents are generally available in one of two configurations—fully or partially covered. A fully covered stent typically includes a continuous membrane over the entire surface of the stent structure. A partially covered stent may include membrane covering over a central portion of the structure but have uncovered ends or flanges. Fully covered stents may be preferred where there is a high risk of tumor ingrowth or overgrowth, or where there is need to seal fistulae. However, because tissue is generally unable to grow into fully covered stents, they are prone to proximal and distal migration subsequent to implantation. The membrane covering also makes the stent more rigid, which further contributes to migration incidence because the encased structure may not sufficiently adapt to temporary changes in lumen shape (e.g., from external trauma or peristaltic motion). Partially covered stents are generally more flexible than their fully covered counterparts and therefore may better respond to such changes. The partial cover design also allows a certain amount of tissue ingrowth, better securing the stent at the site of implantation. Excessive tissue ingrowth and overgrowth can, however, lead to a loss of the stent's functionality and may complicate or entirely prevent stent repositioning or removal. Partially covered stents are also generally ineffective at sealing or mitigating formation of fistulae in the portions lacking membrane coverage.
Whether a fully covered or partially covered stent is selected for a particular procedure often depends on the organ targeted, the particular characteristics of the diseased vessel, the patient's needs, and on physician preference. Despite the aforementioned options, there exists a need for stents that incorporate the advantages of fully and partially covered stents but lack or mitigate the disadvantages of each. In particular, there is need for a covered stent that can prevent excessive tissue ingrowth/overgrowth similar to a fully covered stent, but possess sufficient flexibility to adapt to changes in lumen shape similar to a partially covered stent.